I'm not sure about the limits of the engine's complexity, but the game "shadows of doubt", a procedurally generated murder mystery game, has a giant sandbox with characters that have jobs, partners, visit restaurants, and more.
I don't think the NPCs can do more than a handful of actual actions, but the way you can find who a character met by watching the security tapes of a particular restaurant from a particular time because on of the bat's neighbours says "I saw this person here last night" when you ask about your murder victim is extremely impressive.
There is definitely a sense that you've seen everything after a while because of the limitations of procedural generation, of course, but a sandbox like that combined with scripted quests would make for some really fun gameplay outside of the main quest.
I don't think the NPCs can do more than a handful of actual actions, but the way you can find who a character met by watching the security tapes of a particular restaurant from a particular time because on of the bat's neighbours says "I saw this person here last night" when you ask about your murder victim is extremely impressive.
There is definitely a sense that you've seen everything after a while because of the limitations of procedural generation, of course, but a sandbox like that combined with scripted quests would make for some really fun gameplay outside of the main quest.